A coupled waveguide represented by a directional coupler is an important element which realizes various waveguide type optical devices such as not only an adder/divider, optical modulator and optical switch but also, combining with a diffraction grating, wavelength selecting optical add/dropping multiplexer (optical ADM element), either of the common directional coupler type or of the different directional coupler type, corresponding to the cycle of the diffraction grating. Known as practically indispensable three-dimensional waveguide structures are ridge type structures and buried type structures. In both structures, two waveguides having substantially the same aspect ratio, namely, the ratio between equivalent horizontal and vertical waveguide widths relative to the substrate, are symmetrically disposed close to each other.
For example, for simplicity of fabrication, each of two waveguides in a conventional structure has a transversely flat configuration with a horizontal effective waveguide width of 1 to 3 .mu.m and a vertical effective waveguide width of approximately 0.5 .mu.m. If the vertical effective waveguide width relative to the horizontal effective waveguide width with respect to the substrate is defined as the aspect ratio, then the aspect ratio is 1 or less.
As an example having an asymmetric waveguide structure, a polarization-independent wavelength filter capable of stably selecting a specific wavelength from input light having an arbitrary polarized state is disclosed in Japanese Patent Laid-open Publication 8-184789. In this wavelength filter, two waveguides 11, 12 are disposed closely, a common-directional coupler grating 13 is formed in the input-side polarization separation region, and a wavelength selection region having a DBR- or DFB-structured grating 14 is formed next to the grating 13. In the polarization separation region, widths and thicknesses of the waveguide 11, 12 are determined so that the TE mode propagation constant of the waveguide 11 (wavelength .lambda.1) coincides to the TE mode propagation constant of the waveguide 12 (wavelength .lambda.1). In the polarization selection region, widths and thicknesses of the waveguides 11, 12 are determined so that the TM mode propagation constant of wavelength .lambda.1 of the waveguide 11 coincides to the TE mode propagation constant of wavelength .lambda.1 of the waveguide 12.
In this structure, TE mode components of wavelength .lambda.1 move to the waveguide 12 and TM mode components travel along the waveguide 11 in the polarization separation region. In the wavelength selection region, optical signals with wavelength .lambda.1 travelling along the waveguides 11, 12 are selectively amplified. Since the TM mode propagation constant of wavelength of the waveguide 11 is made to coincide with the TE mode propagation constant of wavelength .lambda.1 of the waveguide 12 in the wavelength selection region, no time difference occurs between waves with wavelength .lambda.1 travelling in both waveguides 11 and 12. By multiplexing outputs from the waveguides 11, 12 in their polarized states, wavelength .lambda.1 can be extracted from input light without dependence on polarization.
In the conventional symmetric coupled waveguide structure, however, the TE mode propagation constant is larger than the TM mode propagation constant in both waveguides. This causes a difference in the coupling length for the TE--TE mode coupling and TM--TM mode coupling between both waveguides, as well as a difference in coupling characteristics, such as half wavelength voltage in case of an optical switch, and a difference in optical filter characteristics, such as center wavelength in case of a diffraction grating loaded structure. That is, the conventional waveguide structure depends on polarization.
Due to the polarization dependency, the conventional coupled waveguide structure is subject to output fluctuation when the polarization of input light varies randomly, for example, in optical fiber communication and this is a practically serious problem.
Moreover, the prior art disclosed in the above-identified publication needs gratings 13, 14 having desired grating constants, respectively, in the polarization separation region and the wavelength selection region. Additionally, the structure must be designed and formed so as to ensure TE--TE mode coupling (or TM--TM mode coupling) between waveguides 11, 12 in the polarization separation region while prohibiting TE--TE mode coupling (or TM--TM mode coupling) in the waveguides 11, 12 in the wavelength selection region, and selection of waveguide parameters is not easy.